fix: resolve golangci-lint violations (errcheck, unused, gosimple, ineffassign)

errcheck: add proper error handling with slog.Error for tx.Rollback, key generation, and s3/dav operations. Add nolint comments for intentionally discarded DB scan errors and HTTP write errors.
unused: remove dead code (svcRow type, processImage, nonDepositPaymentType, generateSecureCode, colorBold, nGreen, nRed)
gosimple S1021: merge var declaration with assignment in manage.go
ineffassign: remove dead assignments in settings.go, till.go, images.go

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/mw/response.go

@@ -11,7 +11,7 @@ import (
 func RespondJSON(w http.ResponseWriter, status int, data any) {
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(status)
-	json.NewEncoder(w).Encode(data)
+	_ = json.NewEncoder(w).Encode(data)
 }
 
 // RespondError writes a JSON error response with the given status code and message.